For a current telephone, a central processing unit (CPU) is used to control the telephone. The great performance of a CPU enhances the functions and applications of a telephone. Among the work terms of the CPU, one of them is to control the communication of the telephone with a telephone switchboard of the telephone company.
Please refer to FIG. 1 which is a schematic block diagram showing a conventional telephone. The telephone shown in FIG. 1 includes a hook switch 11, a state detector 12, a CPU 13, a control circuit 14, an electronic switch 15, a voltage regulator 17, a battery 18, and a downstream circuit 16 including a plurality of devices such as a speech processor, a voice amplifier, etc. When a phone call is made, a ringing signal is transmitted from the telephone switchboard 10 to the target telephone 1 to inform a user of the coming phone call.
When the user picks up the receiver (not shown) of the telephone, the hook switch 11 is conducted to be short-circuited so that the state detector 12 will inform the CPU 13 of the off-hook state. The CPU 13 then has the control circuit 14 conduct the electronic switch 15 to transmit speech signals from the switchboard 10 to the downstream circuit 16. Meanwhile, a power signal accompanying the speech signals and transmitted along the telephone line will be stabilized through the voltage regulator 17 as a working power signal, and fed to the CPU 13. The working power signal enters the downstream circuit for further conduction, and provides stable power for the CPU 13. The battery 18, however, provides power for the CPU 13 to monitor the hook state before the power transmitted along the telephone line achieves the CPU 13.
In the conventional telephone, it is necessary for the CPU 13 to keep an eye on the state of the hook switch 11, and the power needed for such a frequent monitoring operation is provided by the battery. Consequently, the life span of the CPU 13 will be quite limited, and the CPU will not be able to activate the electronic switch 15 if the battery 18 is power-empty.